Conventional passive cooling systems are usually based on convection flow of a fluid in a heat transfer loop, and generally operate when the temperature of an enclosure exceeds the temperature of an external heat sink to which heat is to be rejected. If it is desired that the system should stop operating when the enclosure temperature falls below the particular temperature, active devices, such as valves, motors, or operators have been required to achieve the objective.
The temperature initiated passive cooling system (TIPACS) described hereinbelow cools an enclosure whose temperature has risen above a particular temperature, rejecting the heat to an external environment; the cooling process stops when the enclosure temperature has fallen below a particular temperature. TIPACS, must, while remaining automatic and passive, requiring no valves, motors, or operators to start (initiate), operate, or stop the cooling process, be temperature initiated, operating only when the enclosure temperature exceeds a particular temperature. The particular temperature should be presetable to closely match a preferred temperature. The system must operate independently of external environment temperature; the temperature above which TIPACS operates is independent of the temperature of the external heat sink to which heat is to be rejected. The system should stop operating when the enclosure temperature falls below the particular temperature.
A cooling system having such characteristics is particularly useful for heat removal in situations where extremely high levels of reliability are desired and where overcooling must be avoided. Such situations are found in nuclear reactor systems, chemical process systems, and other heat-sensitive systems. Examples are described hereinbelow.
TIPACS can be used in multiple safety systems for nuclear reactors of all types. Two generic applications are contemplated, and can be employed together or separately.
1. TIPACS can be used for reactor containment cooling. Most modern reactors are located inside containment buildings. Containments are air-tight structures designed to prevent release of radionuclides to the environment in the event of an accident. For example, the containment building at Three Mile Island prevented the large-scale release of radioactivity during an accident in 1979, while the lack of containment building at Chernobyl resulted in the disaster at that site in 1986.
In the event of a nuclear reactor accident, large quantities of steam and/or other hot materials may be released into the containment. If the containment atmosphere is not cooled, the high temperature atmosphere will eventually destroy containment integrity and allow release of radioactivity to the environment. To prevent such a disaster, current reactor containments have active cooling systems with pumps, valves, automation equipment, etc. controlled by operators. A major safety concern is the reliability of such systems. At both the Three Mile Island and Chernobyl accidents, operator errors were deemed major contributors to the accidents.
The most advanced containments overcome some of the problems associated with active cooling systems by the use of passive cooling systems. Typical passive cooling system designs involve constructing the containment as a steel shell which can conduct heat from inside the containment through the steel and reject the heat to naturally circulating or convecting air flowing over the outside of the containment. A major problem with many passive systems is they operate continuously, and often cannot be shut off or stopped from operating when they are not needed, causing overcooling of the containment. For instance, on a cold winter day, if the power reactor is down for maintenance and not producing heat, the containment may be overcooled. Freezing water may damage equipment and make maintenance operations difficult. Blankets or other temporary devices can be used to slow natural airflow by the containment and thus help keep the containment warm, but these are not passive devices. If an accident occurred, they would have to be rapidly removed.
2. TIPACS can be used for reactor core cooling. Several advanced concepts such as Process Inherent Ultimate Safety Pressurized-Water Reactor (PIUS), Advanced Liquid Metal Reactor (ALMR), and Modular High-Temperature Gas-Cooled Reactor (MHTGR), indirectly use passive air cooling for emergency reactor core cooling to prevent core melt accidents. Because these passive air-cooling systems operate continuously, there are problems with over cooling the reactor when it is shut down on very cold days, and excess energy loss to the environment during normal power operations. Typically, about 0.5% of heat loss is due to passive cooling systems during normal operations. With a passive cooling system that operates only above a preset temperature, as described hereinbelow, these heat loses can be minimized.
TIPACS is directly applicable to the control of exothermic chemical reactions in chemical processes. In a typical exothermic chemical process, several chemicals react together to produce one or more desired products. The rate of chemical reaction is generally strongly temperature dependent. If the temperature is too low, the chemical reaction rate may be very show. If the temperature is too high, a runaway exothermic chemical reaction may occur, producing different chemical products, or, in the worst cases, destroying the process equipment and possibly the entire facility. Since TIPACS is a passive cooling system which becomes operational at a preset temperature, it provides a most suitable means for controlling chemical reactor temperatures. It provides both a safety enhancement and an economic advantage.
Historical examples demonstrate the need for TIPACS to control chemical reactor temperatures. For instance, one of the largest chemical plant disasters in history occurred at Seveso, Italy on Jul. 10, 1976. A chemical plant at Seveso used a batch chemical reactor to produce various chemicals. In the process, chemical reactants were added to a reactor tank which was heated until reactions started. The tank was then cooled, using an active cooling system, to control the chemical reaction rate until the chemical reactions were complete.
The accident occurred during a weekend shut down. A batch of chemicals had been loaded into the chemical reactor and partially reacted. Since the plant normally closed for the weekend, the chemical reactor was shut down by over cooling it until the chemical reactions stopped. The active cooling system was shut off and the workers went home. Heat leaked into the chemical reactor from some source, and the exothermic chemical reactions started again, producing added heat. A runaway exothermic chemical reaction occurred, different chemical products were produced, the pressure relief valves opened, and large quantities of dioxin were spread over the community of Seveso. If a TIPACS had been in place to cool the reactor, the accident would not have occurred.
The most common man-made chemical systems in use worldwide are electric batteries. Generally, at low temperatures, the electrical output of most batteries is low. At high temperatures, battery lifetime is limited. At very high temperatures, batteries often undergo selfdestruction, sometimes with explosive force. TIPACS creates the option to thermally insulate a battery to maintain an optimum temperature in cold weather while cooling the battery when the temperature exceeds the optimum operating temperature. Typically, rapid battery discharge or recharge increases battery temperature. This option is particularly important with advanced batteries for electric utility load leveling and for electric drive equipment and vehicles such as forklift trucks and electric cars. In automobiles equipped with internal combustion engines, battery requirements are relatively small, making it economical to oversize the batteries for cold weather. With electric vehicles, however, the cost and weight penalty of oversized batteries becomes very high. Moreover, for advanced batteries such as sodium sulfur, battery performance may require that certain minimum temperatures be maintained.
In many mechanical systems, there is need of cooling accompanied by a need to avoid overcooling. An example is the compression of gases. A gas compressor increases both the pressure and the temperature of the gas. For many applications, gas temperatures must be controlled between a minimum and a maximum. TIPACS provides such a mechanism.
An example of a specific system where TIPACS could be applied is a gaseous diffusion uranium isotope enrichment plant. A typical plant has at least 1000 uranium hexafluoride gas compressors. The gas must be cooled after leaving each compressor. If the gas is too hot, plant efficiency is reduced. If the gas is too cold, it will condense as liquid or solid. With TIPACS, an air-cooled gaseous diffusion plant is possible with no concern about over-cooled or under-cooled gases. The same concept applies to gas compressors in any process where there is an optimum gas temperature.
Another application of TIPACS is in houses and other buildings to minimize heat loss in winter, while providing summer cooling, thus replacing attic fans.
Conventional passive cooling systems comprise a single phase, natural circulation heat exchange loop which removes heat from a warmer enclosure to a cooler external environment. The heat transfer loop generally comprises a first heat exchanger inside the warmer enclosure, and a second heat exchanger in the cooler external environment. The two heat exchangers are generally interconnected with the second heat exchanger higher in elevation than the first heat exchanger.
The passive heat transfer loop being filled with a single phase fluid, heat is transferred from inside the enclosure to the external environment. The warmer enclosure heats the fluid inside the first heat exchanger. As the fluid is heated, it becomes less dense and rises, flowing to the second heat exchanger, where it is cooled as heat is rejected into the external environment. The cooling increases fluid density and the fluid flows downward back to the first heat exchanger.
The disadvantage of the conventional passive cooling system is that it operates constantly when the enclosure temperature is greater than the external environment, and an active system component is required to deactivate and reactivate the system when a desired enclosure temperature is reached.